Elastic nonwoven made from thermoset fibers

ABSTRACT

The present invention is directed toward a nonwoven fiber web material comprised of thermoset fibers that has a machine direction and a cross direction with elastic stretch and recovery properties in the cross direction.

This application claims the benefit of priority of U.S. Provisional Application No. 61/893,963 filed Oct. 22, 2013, all of which is incorporated herein by reference in it's entirety.

FIELD OF THE INVENTION

This invention relates to a nonwoven fibrous web material that has elastic stretch and recovery properties in one direction and is made from thermoset fibers, but includes no inherently elastic materials such as elastomers.

BACKGROUND

Elastic nonwovens are widely known in industry. These nonwovens typically include elastomeric polymers in the forms of fibers, foams and/or films. Others include bicomponent or other thermoplastic polymer fibers that have inherent elastic stretch and recovery properties. For example, a nonwoven/film composite material is described by Middlesworth et al. in U.S. Pat. No. 6,537,930. In addition to elastic nonwovens, anisotropic extensible nonwovens are also well known. These nonwovens exhibit large differences in elongation to break between the machine and cross directions. The elongation, however, may or may not be highly recoverable after extension to values less than the break elongation. Examples of extensible nonwovens and methods to create them are described by Martin in U.S. Patent Application Publication No. 2007/0254545 and by Hassenboehler in U.S. Pat. No. 5,730,923. These examples are also comprised of either elastomeric or thermoplastic fibers. Applications for these materials are limited to those that do not require high temperature stability due to the nature of the polymers employed. In contrast to elastomers and thermoplastics, thermosets are generally known as rigid polymers, with materials such as polyimide possessing high temperature stability. Elastic nonwovens with high stretch and recovery properties made from rigid thermoplastic polymers are unknown. Thus, a need still exists for elastic nonwoven materials that have high thermal stability.

SUMMARY

The present invention is directed toward a nonwoven material comprising thermoset polymer fibers that has a machine direction and a cross direction with elastic stretch and recovery properties in the cross direction.

The present invention is further directed toward a process for making the nonwoven material comprising spinning fibers from a thermoset polymer into an unstretched nonwoven material and then stretching the unstretched nonwoven material to create a nonwoven material.

The present invention is still further directed to a filter media or garment comprising at least one layer of the nonwoven material.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a scanning electron microscope (SEM) image of the as-spun random polyamic acid (precursor) web of the Example.

FIG. 2 is an SEM image of the aligned polyimide web of the Example.

DETAILED DESCRIPTION

Herein disclosed is an invention of a thermoset nonwoven that has highly elastic stretch and recovery properties in one direction. In a preferred embodiment the thermoset is a polyimide. In a more preferred embodiment the elastic nonwoven has a mean fiber diameter of less than 1 micron. The anisotropic properties of this nonwoven are remarkable in that the elongation to break in the cross direction is approximately thirty times the elongation to break in the machine direction. In addition, extensions of the material in the cross direction of less than the break elongation are highly recoverable, with recovery of 90% or more at extensions up to 50% and recovery of 80% or more at extensions up to 75%. In a more preferred embodiment, a recovery of 95% or more at extensions up to 50% and a recovery of 85% or more from extensions up to 75%. In a most preferred embodiment, a recovery of 97% or more from extensions up to 50% and a recovery of 90% or more from extensions up to 75%. In addition to the unique elastic properties, this nonwoven can be comprised of microfibers (fibers less than about 10 microns in diameter) and/or nanofibers (fibers less than 1 micron in diameter).

For the purposes of the present invention, the ISO 9092 definition of the term “nonwoven” article shall be used: “A manufactured sheet, web or batt of directionally or randomly oriented fibers, bonded by friction, and/or cohesion and/or adhesion, excluding paper and products which are woven, knitted, tufted, stitch-bonded incorporating binding yarns or filaments, or felted by wet-milling, whether or not additionally needled. The fibers can be of natural or manufactured origin. They can be staple or continuous filaments or be formed in-situ.”

Also disclosed is a method to manufacture the elastic thermoset nonwoven with fibers of mean fiber diameter of less than 1 micron. The process comprises spinning a fibrous nonwoven material from a polyamic acid and then simultaneously stretching and imidizing the polyamic acid to create a polyimide nonwoven.

Nonwoven fabrics of continuous filaments, such as spunbonds, typically have a more or less random laydown of fibers. The random pattern of the precursor nonwoven for this invention is evident in the scanning electron microscope image of the fabric surface in FIG. 1. This random pattern can sometimes be altered through post processing operations such as stretching which creates anisotropy in the fabric. This anisotropy is evidenced by a preferential alignment of the filaments. FIG. 2 clearly shows the alignment of filaments in the nonwoven of this invention. When anisotropy in fiber alignment is created, differences in mechanical properties, such as tensile strength and elongation to break, between the machine and cross directions result. These differences are typically less than 5:1. That is, for example, that the machine direction break strength might normally be expected to be less than 5 times higher than the cross direction break strength. Alternatively, the cross direction elongation to break might normally be expected to be 5 times higher than the machine direction elongation to break. In a preferred embodiment, the nonwoven of this invention has a surprisingly high difference in break strength ratio of approximately 10:1 (machine direction:cross direction). And an even more surprising 30:1 ratio of break elongation (cross direction:machine direction). These ratios can be seen by comparing the data between Tables 1 and 2. A surprising property of the nonwoven of this invention is the high amount of elastic recovery from extensions up to as high as 75%. A summary data table is shown in Table 3. This table shows that the nonwovens of this embodiment recover 97% of their elongation when stretched by 50% or less and more than 90% recovery when stretched by 75%. Data in the table are the average of 4 test specimens at each extension amount. Elastic recovery rates this high would normally be expected only from elastomeric materials.

Elastic nonwovens are useful for articles such as filter media and apparel. In a filter, the elastic property facilitates the removal of particulate cake by pulsation. In a garment, the elastic property provides ease of movement and comfort.

The present invention is directed toward a nonwoven material comprising thermoset polymer fibers that has a machine direction and a cross direction with elastic stretch and recovery properties in the cross direction.

The thermoset polymer can be polyimide. Also, the polyimide can be fully aromatic.

The thermoset polymer fibers have a mean fiber diameter less than about 2 microns or even less than about 1 micron.

The nonwoven material recovers at least about 95% of the elongation when stretched in the cross direction to at most about 50%. Also, the nonwoven material recovers at least about 85% of the elongation when stretched in the cross direction to at most about 75%.

The nonwoven material has an elongation to break of greater than about 100% in the cross direction. Also, the elongation to break in the cross direction is greater than about 10 times the elongation to break in the machine direction. The nonwoven material has a load to break in the machine direction greater than about 5 times the load to break in the cross direction.

The present invention is further directed toward a process for making the nonwoven material comprising spinning fibers from a thermoset polymer into an unstretched nonwoven material and then stretching the unstretched nonwoven material while simultaneously elevating the temperature above the setting temperature of the polymer to create a nonwoven material. The fibers can be spun from polyamic acid and the unstretched nonwoven material is stretched while simultaneously imidizing the polyamic acid to create a polyimide nonwoven material. The present invention is still further directed toward a filter media comprising at least one layer of the nonwoven material. Also, the thermoset polymer can be polyimide.

The present invention is still further directed toward a garment comprising at least one layer of the nonwoven material. Also, the thermoset polymer can be polyimide.

TEST METHODS

Fiber Diameter was measured using scanning electron microscopy (SEM). In order to reveal the fiber morphology in different levels of detail, SEM images were taken at nominal magnifications of 100×, 1000× and 2500×. For fiber diameter counting, fibers were counted from at least 5 images at a magnification of 2500× using ImageJ software. The fiber diameters were measured by marking the distance between the edges of a fiber in focus and measured using the “measure command” of ImageJ. Each fiber was counted only once. Two hundred fibers were individually marked and the average of the measurements is reported.

Break Load and Elongation: Specimens were tested in a standard atmosphere, maintained at 23±2° C. and 50±5% relative humidity, in accordance with ASTM standard D 1776. Rectangular specimens, measuring 12.7 cm by 2.54 cm, were stamped out from the master roll and allowed to reach moisture equilibrium.

Cross Direction Testing: An Instru-Met refurbished Instron Load Frame model 1123 equipped with a SN695-B 2000 gram load cell and 22.68 kg pneumatic grips, was used for machine direction tensile testing. MTS Corporation Testworks 4® software was used for electronic data collection. The gauge length was set to 5.08 cm. For cross direction tensile testing 4 samples were loaded to breaking point at 50% strain rate, 3.81 cm/min, and the average breaking load and elongation were recorded.

Machine Direction Testing: Due to the increased strength of the specimens in the machine direction a SN749-C 22.68 kg load cell and 90.72 kg pneumatic grips were used. All previous testing parameters were held constant. For machine direction testing 5 samples were loaded to breaking point at 50% strain rate, 3.81 cm/min, and the average breaking force and elongation were recorded.

Stretch Recovery of an elastic polyimide was tested in the cross direction. ASTM methods D 2594-04 and D 3107-07 for stretch properties of knitted and woven fabrics were adapted for the testing procedure. Specimens were tested in a standard atmosphere, maintained at 23±2° C. and 50±5% relative humidity, in accordance with ASTM standard D 1776. Rectangular specimens, measuring 12.7 cm by 2.54 cm, were stamped out from the master roll and allowed to reach moisture equilibrium. As outlined in ASTM methods D 2594-04 and D 3107-07, 5.08 cm benchmark lines were measured and marked in the center of the test specimens.

An Instru-Met refurbished Instron Load Frame model 1123 equipped with a SN695-B 2000 gram load cell and 22.68 pneumatic grips, was used for tensile testing. MTS Corporation Testworks 4® software was used for electronic data collection. The distance between the pneumatic grips was set to 5.08 cm to mimic the distance of the benchmark lines. Specimens were placed between the grips and benchmark lines were visible and the interface of the specimen and grip. The benchmark distance was verified once the specimen was secured.

Prior to testing for stretch recovery, tensile tests were completed to establish the breaking stress of the polyimide. Four specimens were loaded to the breaking point at 50% strain rate, 3.81 cm/min, and the average breaking load and elongation were recorded. After the breaking load was established, stretch recovery testing at 25, 50, and 75% of the absolute elongation was conducted. For each test at the specified elongations, specimens were loaded at 50% strain rate, 3.81 cm/min, until the desired elongation was reached. A hysteresis test was performed during the relaxation of the specimen for one stretch cycle. After testing, the specimens were placed on a general purpose fluorinated ethylene propylene (FEP) film an allowed to recover. Per ASTM methods D 2594-04 and D 3107-07, the specimens were allowed to recover for 30 s, 30 m, 1 hr, and 2 hrs. At each of the time intervals listed, the distance between the benchmark lines was recorded. Calculations for stretch recovery were completed as listed in ASTM method D 2594-04.

EXAMPLE

A highly elastic nonwoven was created by the following process. A polyamic acid fiber precursor nonwoven fabric was formed by electroblowing a polyamic acid solution comprising Pyromellitic dianhydride (PMDA), oxy-dianiline (ODA) and dimethylformamide (DMF) in a manner similar to the process described in U.S. Pat. No. 7,618,579 and U.S. Patent Application No. 2011/0144297 A1.

The polyamic acid solution used to prepare the precursor nonwoven was comprised of 0.965 molar ratio of PMDA to 1 mole of ODA, in a solution of 76.5 weight percent of DMF. The nonwoven was prepared by loading 6 kg of the polymer solution into a Hoke cylinder. A pressurized nitrogen supply was used to distribute the solution to three side by side 10 cm wide electroblowing spinnerets having 3 nozzles each with a diameter of 0.38 mm and length 3.8 mm, arranged 1 cm apart, centered in the spinneret. Heated, compressed air was fed into the spinnerets and was ejected through slots. The fibers were both blown by the air and attracted by a DC voltage electric potential to a metallic rotating drum collector and then peeled from the drum to a wind-up in a continuous process. Heated, pressurized air was also blown via two auxiliary blowers into a Plexiglas® enclosure containing the spinning apparatus and the drum collector. An exhaust blower was used to maintain atmospheric pressure inside the enclosure and remove all evaporated solvent. The distance from the nozzles to the drum collector was 36 cm and the potential difference applied between the nozzles and the drum was 110 kV. Solution feed pressure was 8.69 bar and process gas flow was 1.38 std cu meters per minute at a temperature of 23° C. One auxiliary air supply was heated to 99° C. and blown into the lower portion of the fiber spinning chamber at a flow rate of 0.95 std cu meters per minute. The second auxiliary air supply was heated to 130° C. and blown into the upper portion of the fiber spinning chamber at a flow rate of 0.891 std cu meters per minute.

The uncalendered precursor nonwoven was then trimmed to a width of 39.4 cm wide to remove low basis weight edges. It was then unwound and imidized by pulling it through an infrared oven with 6 zones of infrared emitter pairs above and below the web with an emitter to emitter distance of 23 cm. The unwind was equipped with a hand operated friction brake set to eliminate sagging in the web. Each infrared oven zone was 20 cm long. The center of the oven had a 20 cm space with no emitters where there was a heated air inlet diffuser. Heaters were set to the following temperatures: zone 1 at 350° C., zone 2 at 450° C., zone 3 at 500° C., air inlet at 300° C., zone 4 at 550° C., zone 5 at 600° C. and zone 6 at 650° C. The web was rewound at the exit of the oven with a winder driven at 6.8 feet per minute. The winder was equipped with a Magpower global series magnetic particle clutch model GCD90. The torque control on the clutch was an analog rheostat with 0-10 scale set to 1¾. The web was wound onto an 8.9 cm outer diameter cardboard tube. According to the Magpower website, this clutch is designed to provide a linear torque between 0 and 75.6 Newton-meters at full scale. Therefore, at 1¾ analog setting, the corresponding torque would be approximately 13.2 N-m. At the 1¾″ outer radius of the cardboard tube, the tension on the web would be approximately 300 Newtons. The width of the web at the winder was 13.3 cm.

The precursor polyamic acid nonwoven had an areal basis weight of 21.8 grams per square meter (gsm) and a mean fiber diameter of 981 nanometers. The final, elastic polyimide nonwoven had an areal basis weight of 28.7 gsm. Porosity of the final nonwoven ranged from 88 to 90%. The mean fiber diameter of the elastic polyimide nonwoven was 779 nanometers. Mechanical properties for this Example are shown in Tables 1 and 2. Elastic properties for this Example are shown in Table 3.

TABLE 1 Means of Stress/Strain Data in the Cross Direction for the Example Tensile Toughness Modulus @ Stress Stress Strain Strain Energy (Work Width Thickness 50% Strain @ Yield @ Break @ Yield @ Break Break To Break to Break) cm mm Rate kgf/cm² kgf/cm² kgf/cm2 % % Load gf kg*mm kgf*mm/mm³ 2.54 0.178 11.54 7.457 7.322 164.42 165.55 330.7 12.316 0.036

TABLE 2 Means of Stress/Strain Data in the Machine Direction for the Example Tensile Toughness Modulus @ Stress Stress Strain Strain Energy (Work Width Thickness 50% Strain @ Yield @ Break @ Yield @ Break Break To Break to Break) cm mm Rate kgf/cm² kgf/cm² kgf/cm2 % % Load gf kg*mm kgf*mm/mm³ 2.54 0.178 3333.85 77.409 77.373 3.96 3.97 3494.5 5.235 0.015

TABLE 3 Averages of Stretch and Recovery Data for the Example Applied Elongation (%) 25 50 75 Applied Force (gf) 18.75 33.5 51.15 Original Benchmark Distance (cm) 5.08 5.08 5.08 Distance at 30 seconds (cm) 5.156 5.232 5.486 Distance at 30 minutes (cm) 5.131 5.156 5.41 Distance at 1 hour (cm) 5.131 5.156 5.334 Distance at 2 hours (cm) 5.131 5.156 5.334 Distance at Max Load (cm) 6.35 7.62 8.89 Fabric Growth at 30 sec (%) 2 3 8 Fabric Growth at 30 min (%) 1 2 6 Fabric Growth at 1 hr (%) 1 2 5 Fabric Growth at 2 hrs (%) 1 2 5 

What is claimed is:
 1. A nonwoven material comprising thermoset polymer fibers that has a machine direction and a cross direction with elastic stretch and recovery properties in the cross direction.
 2. The nonwoven material of claim 1, wherein the thermoset polymer is polyimide.
 3. The nonwoven material of claim 2, wherein the polyimide is fully aromatic.
 4. The nonwoven material of claim 3, wherein the thermoset polymer fibers have a mean fiber diameter less than about 1 micron.
 5. The nonwoven material of claim 1, wherein the nonwoven material recovers at least about 95% of the elongation when stretched in the cross direction to at most about 50%.
 6. The nonwoven material of claim 1, wherein the nonwoven material recovers at least about 85% of the elongation when stretched in the cross direction to at most about 75%.
 7. The nonwoven material of claim 1, wherein the nonwoven material has an elongation to break of greater than about 100% in the cross direction.
 8. The nonwoven material of claim 1, wherein the elongation to break in the cross direction is greater than about 10 times the elongation to break in the machine direction.
 9. The nonwoven material of claim 1, wherein the load to break in the machine direction is greater than about 5 times the load to break in the cross direction.
 10. A process for making the nonwoven material of claim 1, comprising spinning fibers from a thermoset polymer into an unstretched nonwoven material and then stretching the unstretched nonwoven material while simultaneously elevating the temperature above the setting temperature of the polymer to create a nonwoven material.
 11. The process for making the nonwoven material of claim 10, wherein the fibers are spun from polyamic acid and the unstretched nonwoven material is stretched while simultaneously imidizing the polyamic acid to create a polyimide nonwoven material.
 12. A filter media comprising at least one layer of the nonwoven material of claim
 1. 13. The filter media of claim 12 wherein the thermoset polymer is polyimide.
 14. A garment comprising at least one layer of the nonwoven material of claim
 1. 15. The garment of claim 14 wherein the thermoset polymer is polyimide. 